Possible models for +1 frameshifting resulting from an eight-nucleotide anticodon stem–loop. (A) The quadruplet decoding model posits that insertions in the anticodon stem–loop of a frameshift suppressor tRNA leads to a four-nucleotide anticodon capable of decoding and translocating a four-nucleotide mRNA codon (with the extra nucleotide shown in green). The numbering of the mRNA begins with the first position in the P site. (B) An alternative model is that the nucleotide insertion in the anticodon stem–loop causes a widening of the loop, allowing the anticodon nucleotide 34 to interact with the fourth nucleotide of the A-site codon (green; numbered as 7 in A). (C) In the P-site slippage model, normal decoding in the zero frame occurs in the A site; however, the transition into the +1 frame occurs after translocation to the P site due to a weakened interaction between the anticodon and codon.
